Because of safety issues associated with moving heavy objects, hoisting equipment often includes safety features that limit how high or how low the loading line can travel. The desire is to limit how high the load can travel so that, for example, it is not damaged or hit by the load or the load is not moved too far in one direction. Similarly, it is often desirable to limit how low the hoist can travel. Several methods or devices are known in the prior art for achieving such hoisting limits.
U.S. Pat. No. 7,097,155 teaches a hoist that includes a device for signaling when a preset minimum number of cable windings are left on the winding drum. The device uses an optical sensor to count the number of revolutions of the drum and the direction of rotation. An alarm is activated when only the preset minimum number of winding layers remain on the drum. Because this system requires an optical sensor that keys off of windings on a drum, it does not have application across a broad number of hoists.
U.S. Pat. No. 6,966,544 also keys off of windings on the drum of a hoist, but with a proximity limit switch. A first proximity limit switch is mounted on the frame adjacent to the hoist drum and it senses the presence or the absence of the hoist rope around the drum. When it senses the presence of the hoist rope around the drum at that point, it signals a control to prevent the hoist motor from further rotating the hoist drum in the wind-on direction, thereby preventing further lifting of the load. A second proximity limit switch is positioned adjacent a second point along the drum, and it signals the control to prevent the hoist motor from further rotating the hoist drum in the wind-off direction when it senses the absence of the hoist rope, thereby preventing further lowering of the load. However, this type of configuration is difficult to adjust depending on the desired limits and also requires a winding drum.
Paddle type or block operated limit switches utilize a mechanical actuator on the hoist that activates a switch when the hoist lifting block makes physical contact with the hoist. However, this type of switch is not usually considered adjustable.
A geared limit switch is one that is typically driven by the same shaft that drives the sprocket on a chain hoist or drum on a wire rope hoist. A geared limit switch operates by counting the number of revolutions of the hoist drum, and when a threshold is met a cam or gear actuates a micro-switch and power is cut. Typically there are a pair of micro-switches, one for up or one for down. However, while a geared limit switch can be fitted with different gear ratios to accommodate various lift ranges, the adjustment becomes more sensitive as the gear ratio becomes numerically higher.
The screw type limit switch has a fine thread shaft with a pair of nuts which travel along its length. A micro switch near each end of the threaded shaft provides the limits. While the nuts are adjusted to provide the settings, this device is limited by the length of the threaded shaft.
Thus, it would be beneficial to provide a limiting device for a hoist or winch that has set points that are easily adjustable and can be used with hoists having non-winding drums or sprockets.